Shopify Scripts won't add rates - only adjust or hide that ones that are returned. If you need something to add rates that's either done directly in the Admin or as you've suspected - the CarrierService API.

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Do you know what the input function does next to the code button where you can edit scripts? Is that just for an example or if you add a product or products in that section does that script affect those product(s) and only those produc(s)?

The input button us the UI for swapping between the code view and the section where you can modify the aspects of the cart your testing against. You can add whatever customer, products, etc in that section to make sure your script is running as expected.

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Oh ok so it's just for testing. Understand. We can't do any liquid logic or anyting using this app at all? So if a product is not in the cart or is in the cart this script will or won't fire based on product conditions like a product tag?

Hello, i really like something like a tutorial from scratch about how to create a completily app, from Auth, to manage inside the store, i can pay for it, i really need due to i'm a woocommerce developer and my company is going to migrate to shopify, please help me with this issue.